1. Field of the Invention
This invention relates to a molding elastomeric composition. More particularly it relates to a molding thermoplastic elastomeric composition having superior flexibility, high-impact properties, mechanical strength (particularly, flexural modulus), moldability (particularly, mold-release characteristics) and appearance of the resulting molded products.
2. Description of the Related Art
Heretofore as polyolefin thermoplastic elastomers, there have been known blends of propylene polymers with non-crystalline ethylene-.alpha.-olefin random copolymers or with hydrogenated products of styrene-butadiene-styrene block copolymers (see Official gazette of Japanese patent application laid-open Nos. Sho 50-14742/1975, Sho 52-65551/1977, Sho 58-20664/1983, Sho 58-215446/1983, etc.).
However, elastomeric compositions obtained by blending propylene polymers with hydrogenated products of styrene-butadiene-styrene block copolymers and further hydrocarbon oils for improving the flowability thereof have a drawback that when molded, the mechanical strength (tensile strength and flexural modulus) of the resulting molded products is lower. Further, elastomeric compositions obtained by blending propylene polymers with non-crystalline ethylene-.alpha.-olefin random copolymers have a good rubber elasticity, but on the other hand, they have a drawback that when the compositions are molded, the mold-release characteristics of the resulting molded products are inferior and particularly when the quantity of the ethylene-.alpha.-olefin random copolymers added is large, the resulting molded products have no mold-release characteristics.
Further, an official gazette of Japanese patent application laid-open No. Sho 61-14248/1986 discloses an elastomeric composition obtained by blending propylene-ethylene block copolymers with hydrogenated materials of styrene-butadiene-styrene block copolymers and if necessary, non-crystalline ethylene-propylene rubbers. However, the above elastomeric composition has a drawback that when molded, flow marks appear on the surface of the resulting molded product, and the problem of such an inferior appearance has not yet been solved.
Still further, elastomeric compositions obtained by blending propylene homopolymer with the above-mentioned hydrogenated products of styrene-butadiene-styrene block copolymers and if necessary, non-crystalline ethylene-propylene rubbers have drawbacks that elastomeric compositions having a good elasticity cannot be obtained and the high-impact properties at low temperatures thereof are insufficient. Further, elastomeric compositions obtained by replacing the above-mentioned propylene homopolymer by propylene-ethylene random copolymers, when molded, afford molded products having superior mechanical strength, high-impact properties and appearance, but sink marks or fins are liable to appear on the surface of the molded products so that a problem relative to molding (particularly, mold-release characteristics) has been raised.
The present inventors have made extensive research in order to solve the above-mentioned problems directed to the polypropylene elastomeric compositions, and as a result, have found that the following compositions can solve the above-mentioned problems:
a composition obtained by blending a propylene-ethylene block copolymer (I) with a specified crystalline propylene-ethylene random copolymer (II) within a specified range of ratio and further blending with the resulting propylene copolymer, a specified quantity of a specified hydrogenated styrene-butadiene-styrene block copolymer;
a composition obtained by further blending with the above-mentioned composition, a specified quantity of a non-crystalline ethylene-.alpha.-olefin random copolymer; and
a composition obtained by blending a propylene-ethylene block copolymer (I) with a crystalline propylene-ethylene random copolymer (II) having a specified flexural modulus within a specified range of ratio and further blending with a specified quantity of the resulting propylene copolymer, a specified hydrogenated styrene-butadiene-styrene block copolymer, a non-crystalline ethylene-.alpha.-olefin random copolymer and a higher fatty acid amide, in specified quantities, respectively.